The usefulness of superconducting electronic devices, particularly in field applications where cryogens may not be easily accessible is currently limited by the ability of dewars which hold the liquid cryogens to maintain a superconducting temperature for much more than one week. The existence of long hold time dewars, which can hold superconducting temperatures for 6 months or more, is expected to accelerate the development of devices for laboratory use, for magnetic field measurements, for digital processing using superconducting sensing devices and for other applications.
Superconducting electronic devices, such as superconducting quantum interference devices (SQUID), may be mounted on the outer surface of a chamber, which contains a suitable liquid cryogen, such as helium or nitrogen. Use of SQUID devices in this manner for superconducting magnetic sensors is severely limited by the normal one week dewar hold time. The present invention provides a liquid dewar which may hold liquid cryogens at superconducting temperatures for up to six months or more.